The Darkest Knight was exiled from Meridell. He had built a keep near Market Town and ruled it as a tyrant, though the townsfolk killed him many years ago. Fears of that past later ressurected him through the Darkest Faerie's magic, and he returned to rule the town again, imposing heavy taxes on the locals. Tor and Roberta eventually destroyed him, however, and the town was freed again.
The Darkest Knight had a small cameo in the Petpet Park plot. He appeared out of portals opened in Faerieland. It is unknown if this was a portal in time, or if the Darkest Knight survived his encounter in Market Town somehow.
He is only called the Darkest Knight on the Neopets website. Within the Darkest Faerie game, he is only known as the Black Knight.